Pronounced as “coming soon” for over a decade, the ability to run “last mile” fiber optic cables for communications and data transfer has never actually materialized except in limited field tests. The slow deployment owes to the high component and system costs due to both expensive manufacturing/design techniques and inadequate component performance. Thus, a technically feasible albeit brute force solution has failed to be implemented in the face of economic realities.
Past and present attempts to implement a complete fiber optic network are best summarized as follows:
Initial optical fiber deployments were initially limited to the major “trunk lines” connecting large populations and data sources due to costs for buying, laying, and connecting the fiber to the existing communications infrastructure.
Follow-on deployments saw fiber optic cables extended from these major access points outward to local distribution points, but still not to each individual household.
At present, the industry has used a Passive Optical Network (PON) design which has enabled the amortization of the cost of the expensive optics at the Optical Line Termination (OLT) over several homes, but the number of optical signal splits are limited by the need to deliver Analog Video services over the fiber. The ability to deliver analog video services is required if the optical fiber systems is to compete with the existing cable TV infrastructure.
Pundits and futurists have cited several uses for the impressive data transmission capacity inherent in fiber optic based systems if such a system were broadly deployed all the way to the end user. However, none has proven to be a compelling business market due to present day economics of the required infrastructure. Such things as real time streaming digital video delivered on demand could have a pronounced ability to change or open new markets if only a technical solution could be cost effectively delivered.
Major issues confronting the delivery of these services of a fiber delivery based system include:
Cost of deployment vs return on investment (ROI),
Bandwidth limitations due to passive loss and dispersion in the optical path, and
Number of customers served on a PON due to Analog receiver sensitivity vs carrier to noise ratio (CNR).
Thus, there is a need in the art for systems and methods through which service providers may deploy low cost fiber systems for the mass delivery of the broadband services that the end users desire.
One stumbling block to the deployment of fiber to the home has been lowering the cost of an optical network termination (ONT) in a customer's house. FIG. 3 shows an example of an optical network topology using both powered and passive optical components. One ONT is required at each termination, as shown by the houses in the drawing. FIG. 4 shows a more detailed view of an example termination at a house, with the ONT clearly shown. The ONT in this example includes several parts: an optical receiver, a wavelength-division-multi/demultiplexer (WDM) transceiver module to split a multiplexed signal into discrete channels, interface circuits to the customer terminals, and optionally, a power supply and battery. Typically, the optical receiver is integrated with the WDM transceiver as a single field replaceable optical WDM transceiver module. The module provides a complete interface between the optical transmission world and the electrical transmission world in a single package.
Optical receivers for light guide systems of the type employed in cable systems generally use a high frequency photodiode to convert the light signal to a photocurrent. The photocurrent is proportional to the received intensity of the light signals, and then applies the resulting current to an input circuit.
Since the current obtained from the sensor circuit is often too small to be usefully applied to data recovery circuits, it is desirable to amplify the sensor's photocurrent signal in order to make it relatively insensitive to the introduction of ambient noise during signal processing stages. To this end, optical receivers include a so-called “front end” trans-impedance amplifier (TIA) which raises the level of the signal several orders of magnitude. The output of the front end is then further amplified and shaped in a later section of the signal processing system.
It is desirable for an optical receiver to have a wide dynamic range, both in input intensity and for input frequency. The reasons a wide dynamic range of input intensities is desirable include (1) variations in the assorted cable lengths and multiple types of light sources with which the optical receiver may be used; and (2) variations in light attenuation that can occur with variations in cable lengths, all of which has an effect on light intensity output. However, since the light signals may have significant variations in intensity, resulting in a wide range of input currents, the amplification circuits need to be capable of handling a wide dynamic range of input currents depending on the strength of the received light signal. The received signal strength will vary, for example, as a function of distance from the transmitter, quality of circuit components, number of passive splits, etc. In most cases, the receiving system has no prior knowledge of its distance from the transmitter and topology and it is therefore important that any designs have the flexibility to accommodate the full range of input signal strengths.
An optical receiver with a wide dynamic range of input frequencies may handle additional channels within a multiplexed signal, or may support higher data rates. A limiting factor in the optical receiver's dynamic range is the dynamic range of the trans-impedance amplifier, which is in turn limited by factors such as components selected, the circuit design, and the ambient noise introduced by various circuit components. One such limiting factor is the use of feedback designs, which limit the overall dynamic range of the circuit by introducing stabilization timing constraints within the feedback loop.
In general, analog video delivery over fiber is widely used in the CATV industry to distribute video service between Head-ends, Hubs and Nodes. As such, it would seem to be a normal evolution of system design to extend the distribution of this type of video to the home over a PON system using part of the 1550 nm band.
Today, state of the art video receiver designs allow the ONT/Home Gateway to receive a video signal as low as −6 dBm while still maintaining acceptable CNR and distortion performance over the specified range of received input power. Although this gives performance levels that approach today's cable equivalent, it does come with a price.
Currently, optical video receivers do not use the same method of coupling or signal recovery for analog video transmissions as those used for digital signal transmissions. Digital systems use a high gain Trans-Impedance Amplifier which provides good noise performance but poor linearity for the number of signal channels needed (79 NTSC channels) to satisfy minimum expectations. To effectively compete with the CATV market, an additional 30 or more digital channels will be required to be transmitted above 550 MHz, the current limit.
One of the most efficient designs currently available uses a transformer coupling that matches the amplifier input impedance to the coupling resistor in series with the Photo-Diode as shown in FIG. 2. This type of receiver provides good performance for both CNR (about 48 dB) and Intermod-Distortion (approximately 0 to −5 dBm); and there is an inherent loss due to a maximum power transfer of approximately 3 dB. This loss could be decreased by coupling the current directly in to the amplifier, but linearity then becomes a greater concern at the higher levels in received optical power.
It is additionally desirable to limit the amount of noise introduced into the sensor circuit by the “front-end”. Limiting the amount of introduced noise permits the circuit to operate at lower power and with higher responsiveness, providing advantages such as more efficient signal recovery, reduced operating costs, less heat dissipation, and improved dynamic range.
It will be appreciated that improvements in the art described in the present disclosure that satisfy the above requirements will find use in other fields in which it is necessary to recover a low power signal from a carrier signal, or to amplify a low power signal for further processing.